Technical Abstract:
Theory predicts that plants should close their stomata at night to decrease water loss when photosynthetic carbon gain cannot occur. However, significant nighttime stomatal conductance (g) and transpiration (E) have been measured in a wide range of species and functional groups. Little is currently known about how genetic variation and environmental factors control these physiological traits. To explore the relationship between genotype x environment effects on nighttime water loss, we conducted a glasshouse experiment, measuring nighttime g and E in six clones of the C4 grass, Distichlis spicata, grown under three salinity levels (7, 300, 600 mM NaCl). We hypothesized that under high soil salinity, nighttime g and E would be reduced but that the magnitude of these responses would vary by genotype. We expected genetic variation in these physiological traits, in part, based on variable growth responses to soil salinity among Distichlis clones in field and glasshouse measurements. Despite low vapor pressure deficit (~1 kPA), nighttime g and E were significantly higher than empty chamber measurements (one-tailed t-test, P<0.0001), averaged across all salinity treatments and genotypes (mean g and E of 20 mmol m-2 s-1 and 0.23 mmol m-2 s-1, respectively). Salinity reduced both nighttime g and E by 1.6- and 1.4-fold, respectively. Averaged across salinity concentrations, there was a 1.3-fold difference between genotypes for both physiological measurements. Although genetic variation influenced the magnitude of plant responses to salinity, environmental factors had a stronger effect overall on nighttime g and E. Together, these data suggest that plants can limit nighttime water loss, thus reducing the cost of this trait when water is less physiologically available (i.e., under high soil salinity).